Design of a Helical-Shaped TM₀₁–TE₀₁ Mode Converter Based on Eigenmode Expansion Method

A novel helical-shaped circular TM01–TE01 mode converter working in the X-band is proposed in this article. By combining the polarization conversion of rays in free space and mode conversion in a waveguide, the mode conversion potential between TM<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{0}m}$ </tex-math></inline-formula> mode and TE<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{0}{n}}$ </tex-math></inline-formula> mode of helical corrugated waveguide (HCW) is fully explored. Through the eigenmode expansion method, the process of mode conversion in HCW can be clearly demonstrated, guiding the construction of the final helical-shaped TM01–TE01 mode converter. Simulation results reveal a peak conversion efficiency of 99.6% at 8.6 GHz and a bandwidth of 16.5% with over 95% conversion efficiency (center frequency 9 GHz). In addition, the mode converter has a power capacity of gigawatt level with a longitudinal length of only 3.81 wavelengths. To validate the structure, a prototype was fabricated and measured. The results of the experiment are in great agreement with the simulation, which verifies the effectiveness of this proposed mode converter.